Fluorescein is a known fluorescent agent, that is in the presence of ultraviolet light, solutions of the compound fluoresce, or give off light. This characteristic has been used to examine subterranian waters for sources of underground springs, and detecting sources of contamination of drinking water and infiltration with waste waters of factories. An extension of this use would be to have fluorescein incorporated into biological systems, such as cells to follow the paths of such cells, or to separate certain cells from other cells. However, this has not been possible owing to the non-incorporation or non-selective incorporation of fluorescein into such biological systems. The present invention solves this problem by providing means for incorporating fluorescein into cells, retaining the fluorescent quantities of the compound.